Operators drill wells many thousands of feet in the search for hydrocarbons. The wells are expensive and take a significant amount of time to plan. To obtain hydrocarbons such as oil and gas, boreholes are drilled by rotating a drill bit attached to a drill string. The drill bit is typically mounted on the lower end of the drill string as part of a bottom-hole assembly (BHA) and is rotated by rotating the drill string at the surface and/or by actuation of down-hole motors or turbines.
A substantial number of wells being drilled today have a wellbore path that deviates from the traditional vertical wellbores that were more common before the 1970s. Often, deviated wells are classified as horizontal wells, meaning that such wells have a wellbore path that deviates to horizontal (for example, an inclination of around 70° to 110° from vertical). Often deviated wells are used in unconventional drilling applications such as recovery of hydrocarbons from shale, oil sands, or other alternatives to conventional hydrocarbon reservoirs.
The process of steering a wellbore away from the normal vertical path so that it travels in a desired direction is referred to as “directional drilling.” Most horizontal wells begin at the surface as a vertical well. Drilling progresses until the drill bit reaches a desired depth. At that point the drill bit can be steered using, for example, a mud motor to control the drilling angle. For horizontal wells, the bit will be typically steered to change the wellbore orientation from vertical to a desired deviated angle (such as horizontal) over a distance of several hundred feet. In many cases, straight ahead drilling can be resumed at that point, with various downhole measurement-while-drilling (MWD) instruments used to monitor the drilling path so that additional steering corrections can be made where needed.
Especially for horizontal and deviated wells, it is often difficult to accurately determine the exact trajectory of the wellbore underneath the surface due to inaccuracies in conventional MWD survey data. Accurate knowledge of the location of the wellbore is essential to both hitting the desired target during drilling and avoiding collisions with adjacent wells. Determination of the true vertical depth (TVD) along a wellbore is also needed to allow accurate geological mapping of reservoirs or other geological formations and to fulfill requirements of various regulatory agencies. Because conventional MWD surveys do not provide sufficient positioning accuracy, it is often necessary to conduct a separate final survey of wellbore position, for example using gyroscopic survey tools. This type of additional final survey interrupts the drilling process and thus increases costs.
What is needed is an improved method and apparatus for determining wellbore position in deviated or horizontal wells.
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